Older adults have more nighttime sleep complaints and more objective disturbances of nighttime sleep than young adults. At the same time, older adults nap more during the day. The reason for this apparent change in sleep pattern is not known. The goal of this study is to examine the 24-hour (circadian) pattern of sleep tendency in older adults (over age 70) and younger adults (age 20-30). In order to do this, research subjects are put on a 90-minute day, consisting of 30 minutes of sleep and 60 minutes of wakefulness. They continue this schedule around the clock for 60 hours (2 1/2 actual days), so that we can observe sleep tendency at each point of the body's circadian rhythm.